Ho co-doped TiO2 nanorods

Materials Research Bulletin 118 (2019) 110502

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Synergistic effect of N-Ho on photocatalytic CO2 reduction for N/Ho codoped TiO2 nanorods

T

Xiya Wang, Zhaoguo Zhang, Zhengfeng Huang, Peimei Dong, Xiaoxiao Nie, Zhi Jin, ⁎ Xiwen Zhang State Key Laboratory of Silicon Materials, School of Material Science and Engineering, Zhejiang University, Hangzhou, 310058, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Synergy Holmium Nitrogen Photocatalysis

The photoreduction of CO2 by using Ho and N co-doped TiO2 was explored. The doping elements can further improve the separation efficiency of photogenerated electron-hole pairs via refining grains, causing lattice distortion and adsorbing more hydroxyl radicals(·OH). We found that there exists product selectivity such that the photoreduction products of Ho/N-TiO2 mainly consist of CO and methanol, while the counterparts of pristine TiO2 are CO and methane. The possible mechanism of CO2 reduction is depicted involving the generation and combination of specific radicals. Thus, the amount of ·OH adsorbed on the surface of the catalyst makes a significant difference in the selectivity in the reduction products. Ho/N-TiO2 catalyst with the synergic effect of Ho and N showed the best photoreductive performance for CO2 with H2O to CO and CH4, and its formation rate of CH4 was 29 μmol/gcatal./hour.

1. Introduction With the rapid development of global industries and economies, we are faced with the challenge of the exhaustion of fossil fuel together with severe climatic variation. Photoconversion from CO2 to chemical fuels such as CH4 and CO is an attempt to simultaneously achieve both goals. For one thing, CO2 in air is the most abundant and economic carbon resources in the world. For another, reduction of CO2 to fuels is not only beneficial for reducing CO2 emissions, but also offers a choice of alternative energy resources. Photocatalyzing CO2 with H2O to CO and CH4 is identified as artificial photosynthesis and has already drawn social attention, especially after Fujishima and co-authors reported photocatalytic reduction of CO2 to hydrocarbon compounds [1]. Since then multifarious high-performance photocatalysts for CO2 reduction with H2O to fuels have been reported, such as sulphide, haloid, heterostructure semiconductors and organic polymers [1–6]. Nonetheless, its practical applications are still limited by low conversion efficiency and low formation rate of reduction products due to the rapid recombination rate of photogenerated electron-hole pairs. Hence, there is still a long way to go to obtain an ideal high-performance photocatalyst with suitable band gap and whose separation efficiently is high. Titanium dioxide (TiO2) is one of the most thoroughly investigated semiconductor materials due to its anti-corrosion, environmentally friendly and high photocatalytic activity characteristics [1,3,7–12].

⁎

However, the inherent drawbacks of pristine TiO2 have greatly precluded its practical applications: firstly, its intrinsic photo-response range: rutile (band gap =3.2 eV) can be only photoexcited by UV light which only accounts for about 5% of the solar light spectrum; secondly, the high recombination of its photo-generated electron-hole pairs. Therefore, researchers have been making great endeavors to enhance quantum yield and broaden its spectral response range. Hydrogenation, doping, loading of noble metals and sensitization are common treatments [13–16]. Among the various treatment methods, doping takes effect by adjusting and controlling the position of the valence band and conduction band through changing the crystal structure or by forming defects [17]. Nitrogen is adjacent to oxygen in the periodic table, and its comparable atomic size with oxygen means it needs less activation energy to achieve substitution. Furthermore, it has been proven theoretically and experimentally that N doping TiO2 shows visible-light response without any reduction of photocatalytic activity under ultraviolet light [18–20], which was firstly reported by Asahi as published in Science [21]. Masahiro reported the photocatalytic activities of nitrogen doped TiO2 powders under visible light and have concluded that the isolated N-2p narrow band above the O-2p valence band is responsible for the visible light response, when nitrogen is lightly doped(up to about 1%) into oxygen sites [22]. Above all, N doped TiO2 has a higher OH-index value than undoped TiO2 [23]. It has to be noted that many studies on TiO2 films have emphasized the enhancement of hydroxyl

Corresponding author. E-mail address: [email protected] (X. Zhang).

https://doi.org/10.1016/j.materresbull.2019.110502 Received 27 February 2019; Received in revised form 6 May 2019; Accepted 24 May 2019 Available online 25 May 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.

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2.2. Fabrication of samples

radicals (·OH), because they are the main active species participating in the reaction [24–26]. However, when the doping amount of N is in excess, N species may be packed very closely together on the crystal surface of TiO2 [12]. Thus, increasing the amount of doping N without limit will not necessarily result in better performance. Since mono-doping can accordingly no longer meet the requirements, we attempted herein to dope with two elements in order to acquire superimposed effects. Among the various elements, we ultimately chose holmium (Ho) due to its merits listed as follows. Firstly, the ionic radii of Ti4+ and Ho3+ are 0.074 nm [27] and 0.0908 nm [28], respectively. Obviously, the Ho3+ ion is much larger than the Ti4+ ion, and that Ho3+ replacement of Ti4+ will causes TiO2 lattice distortion and grain refinement which will result in severe property changes. For example, the reduced size of the TiO2 crystal not only results in the increase of specific surface area, but also a lower probability of recombination of photogenerated electron-hole pairs. Secondly, rare earth metal ions are usually employed by researchers as catalysts or to promote catalysis for incompletely occupied 4f and empty 5d orbitals, which can enhance interfacial charge-transfer efficiency [29–31]. Finally, Ho sites can act as electron trapping agents to promote electron-hole separation [30]. Herein, novel photocatalysts of Ho, N and co-doped TiO2 were successfully fabricated by a hydrothermal method. The Ho-TiO2 and NTiO2 photocatalysts show higher CO2 conversion efficiency than the pristine one. Ho/N-TiO2 exhibits the best catalytic performance with its CH4 production greatly exceeding that of the unitary doped system. The mechanism of CO2 conversion with H2O will also be discussed and elaborated upon.

The basic solution was a mixture consisting of DI, HCl, and TBT in a volume ratio of 30:30:1, with which a 30-minute stirring was carried out. According to the required need, specific amounts of mixtures composed of NH4Cl and HoCl3(H2O)6 were added into the basic solution with various atomic ratios of N:Ho:Ti (0:0:300, 3:0:300, 3:1:300, 3:3:300, 1:3:300, 0:3:300), and were labelled as pristine TiO2, N-TiO2, Ho/N-TiO2-Ⅰ, Ho/N-TiO2-Ⅱ, Ho/N-TiO2-Ⅲ, and Ho-TiO2, respectively. The above solutions then underwent a 2 -h vigorous stirring to obtain the hydrothermal solution. Afterwards, the thoroughly rinsed FTO glass was placed into a stainless steel autoclave containing the hydrothermal solution. The hydrothermal treatment proceeded at 120 °C for 24 h. Finally, the resulting samples were completely washed with DI and dried in air.

2.3. Characterization Morphologies of the as-prepared samples were observed on a field emission scanning electron microscope (FESEM, Hitachi S4800). Specific surface areas of the samples were determined by nitrogen adsorption at 77 K using the Brunauer-Emmett-Teller (BET) method on a gas adsorption apparatus (AUTOSORB-1-C, Quantachrome Instruments). X-ray diffraction (XRD) patterns of the samples were recorded on a PANalytical X’Pert PRO X-ray diffractometer using Cu Kα radiation with a 2θ scan range from 20 to 80° at a rate of 4°/min. Backscattering Raman spectra of the samples were obtained on a DXR SmartRaman spectrometer (Thermo Fisher Scientific) using a 532 nm diode laser (400 mW) at room temperature. X-ray photoelectron spectroscopy (XPS) data were collected on a VG ESCALAB MARK II spectrometer with an Al Kα (1486.5 eV) X-ray source, and the spectra were calibrated by the C 1s peak at 284.6 eV of adventitious carbon. Fourier transform infrared (FTIR) spectra were measured on a Bruker Tensor 27 infrared spectrometer between 500 and 4000 cm-1 with samples embedded in pressed KBr discs. Water contact angles were measured by a video-based contact angle measuring device (OCA 20, Dataphysics Instruments GmbH, Germany) at ambient temperature. UV–vis absorption spectra of the samples were obtained on a UV-2600 UV–vis spectrophotometer (Shimadzu Corporation) using a 350 nm excitation wavelength at room temperature. Photoluminescence spectra (PL) of the samples were obtained on a FLS920 fluorescence spectrometer (Edinburgh Instruments Ltd.) at room temperature using 320 nm as the excitation wavelength.

2. Experimental section 2.1. Materials Tetrabutyl titanate (AR, TBT) functioning as the Ti- source, ammonium chloride (AR, NH4Cl) acting as the N- source, and hydrochloric acid (AR, HCl) were purchased from the Sinopharm Chemical Reagent Co., Ltd. China, and used without further purification. Holmium chloride hexahydrate (99.9%, HoCl3·(H2O)6) serving as Ho-source was produced by Aladdin Industrial Inc. China. Fluorine-doped tin oxide (FTO) glass functioning as support was supplied by HEPTACHROMA. Deionized water (DI) was used during the entire process.

Fig. 1. Schematic of the apparatus for the photocatalytic reduction of CO2. 2

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contrast, Ho doped nanorods change from quasi-square tops to shaper, more pointed ones (Fig. 2c and d). In addition, the doped TiO2 also exhibited different increases in the corresponding BET surface area in comparison with the pristine one. The exact figures are listed in Table 2. Apparently, the Ho and N co-doped TiO2 (Ho/N-TiO2) possessed greater nanorod lengths and BET surface areas than those of the pristine TiO2 and mono-doped TiO2 with the exception [35]. As mentioned above, rare earth metal ions can modify the TiO2 crystal structures and properties. Such variation observed herein can be attributed to the anisotropic growth of TiO2 derived from the presence of Ho. The detailed analysis will be shown combining the XRD for all samples below. Overall, the increase of BET surface area indicates more contact area, which is beneficial to the enhancement of photocatalystic reactions [31]. Fig. 3 shows the XRD patterns of the pristine and doped TiO2 samples. It is apparent that both the pristine and the doped TiO2 samples possess a single rutile-TiO2 phase [36]. Compare with the pristine TiO2, the doped TiO2 samples show distinct changes in their corresponding XRD patterns with the variation of dopants, especially the increase of the intensity ratio of the (200) facet to the (101) facet [37], indicating that the introduction of dopants has a definite impact on the crystallization of the resulting material [31,38]. The absence of characteristic peaks of HoCl3 and NH4Cl implies that the N and Ho dopants are incorporated into the lattice. In the precursor, partial TiO6 octahedrals were attacked by Ho3+ and NH4+ ions, and the original Ti atoms of TiO6 octahedrals were substituted by Ho and N, resulting in the formation of the Ho atom-substitutional (HoeOeTi bond) and N atom-substitutional (NeOeTi bond) octahedrals respectively. Based on the fact that the ionic radii of both Ho3+ (0.089 nm) and N3− (0.148 nm) are larger than those of the corresponding substituted Ti4+ (0.053 nm) and O2− (0.121 nm) ions, substitution inevitably induces lattice expansion and distortion. Meanwhile, considering that the degree of similarity of N3- to O2− is better than that of Ho3+ to Ti4+, a larger lattice distortion will take place in the Ho doped TiO2 samples, leading to a smaller crystal size. According to prediction, with the increasing amount of doping with Ho, the size of crystal should have become smaller. In fact, however, there exists a minimum value. Though Ho mono-doped TiO2 contains more Ho, the crystal size is much larger than that of Ho/N-TiO2-Ⅲ. We consider that the reason for this phenomenon lies in the synergistic effect between N and Ho. In other words, Ho plays a dominant role in grain refinement, and N and Ho co-doping can lead to a further decrease of the crystal size. It should be noted that a decreased crystal size indicates more interface area, which can remarkably promote the separation of the photo-generated charge carriers to enhance the photocatalytic performance. Moreover, either the Ho or N doping can damage the intrinsic charge balance of the pure TiO2; thus, oxygen vacancies are generated to restore the overall charge balance. These oxygen vacancies promote the adsorption of eOH on the surface; hence, the photo-generated holes readily react with the surface-adsorbed eOH and form oxidative hydroxyl radicals under light illumination. Therefore, the recombination of photo-generated charge carriers can be greatly restrained, facilitating and enhancing the photocatalytic activity. Furthermore, we also got XRD patterns of used samples, which is almost the same as that of the fresh sample. It affirmed stability of the sample. The strong scattering properties of Raman spectra can be used to further obtain and analyse the phase structure characteristics of the photocatalysts. Fig. 4 exhibits Raman spectra of pristine TiO2, N-TiO2, Ho/N-TiO2 series and Ho-TiO2. The two characteristic Raman bands centered at 444 cm−1 and 610 cm−1, correspond to Eg and A1g modes respectively. Additionally, there is a second-order effect located at approximately 244 cm−1 and a feature situated at 687 cm−1, all of which are assigned to rutile TiO2 [39]. According to previous research, the Raman intensity of TiO2 increases when the particle size is larger, in the low wavenumber region [40]. In this study, it is apparent that the

2.4. Photocatalytic reduction of CO2 Photocatalytic reduction of CO2 was carried out in a quartz reactor as shown in Fig. 1, with the as-prepared samples in fixed position at the center of the reactor. The irradiation was provided by two 300-W (Shanghai Jiguang Co.) visible light sources, which were situated at opposite sides of the reactor. A mixture of CO2 and water vapour acted as the reacting gas, which was pumped into the reactor at a flow rate of 100 ml/min for 30 min in advance in darkness to remove the residuals in the reactor. After 30 min, the visible light sources and the cooling water system were switched on, and the reacting gas was supplied at a constant flow rate of 50 ml/min over the entire measurement time. The experiment was operated for 8 h at room temperature, and data were collected hourly to record the variation of the reduction product concentrations in the effluent gas with illumination time using a gas chromatography system (GC/FID) equipped with an N 2000 dualchannel chromatography data workstation (GC, Zheda Zhida). The measurements of photocatalytic reductions of CO2 were also conducted in the dark to exclude possible interference factors such as extraneous of organic substances. The whole experimentation will be carried out several times to exclude chance factors. Generally, in the presence of a photocatalyst and illumination, the proof of CO2 reduction involves the detection of methane (CH4) or carbon monoxide (CO) in the reduction products. In this study, reduced products, including CH4 and CO were detected. Observations showed that there was no selectivity in the reduction products, and the proportions of CH4 and CO were more than 95% in all samples, in which the ratios of CH4 and CO were analogous. Given that CH4 and CO were consistently the majority of the reduced products, together they can characterize the overall photocatalytic reduction ability similar to that of a single reduction product such as CH4. According to the number of electrons consumed in the formation of the reduced products (Table 1), one unit of CO is equivalent to 1/4 unit CH4. Here, the equivalent total yield of reduction products can be approximately calculated by the following formula: n(total) = n(CH4) + 1/4n(CO), where the n(total), n(CH4), and n(CO) refer to the equivalent total yield of reduced product, the yield of CH4 and the yield of CO, respectively. By weight, the amounts of doped TiO2 photocatalyst (mT) were about 0.02 g with a variation of 5%. The production rate of the total yield can be obtained by the formula: v(total) = n(total)/mT.

3. Results and discussion 3.1. Morphology and properties of photocatalysts In order to investigate the influence of the dopants on the resulting corresponding morphology, the samples were photographed and their FESEM images are displayed in Fig. 2. As shown in the figure, nanorod arrays of all the samples are uniformly distributed on the support with an almost perpendicular orientation with respect to the support. However, the nanorods vary in shape as affected by the corresponding dopants. As shown in Fig. 2, the pristine TiO2 nanorods are cubic columns with quasi-square tops and smooth side surfaces, which is consistent with the growth rule of tetragonal crystal structures [32]. In the meantime, there is no megascopic change in the shape of the N doped TiO2, and similar observations were reported previously [20,33,34]. In Table 1 CO2 reduction potentials converted to SCE reference, pH 7. E0 (V) vs SCE

Reaction (1) (2) (3) (4)

CO2 + 2H+ CO2 + 8H+ CO2 + 6H+ 2H+ + 2e−

+ + + →

2e− → CO + H2O 8e− → CH4 + 2H2O 6e− → CH3OH + H2O H2

−0.77 −0.48 −0.62 −0.65

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Fig. 2. FESEM images of the pristine and doped TiO2 samples, (a) tilted cross-sectional view of pristine TiO2; cross-sectional views of (b) N-TiO2; (c) Ho/N-TiO2-Ⅲ; (d) Ho-TiO2. Table 2 The nanorod lengths, BET surface areas and crystal particle sizes of as-prepared pristine and doped TiO2. Samples

Nanorod length (μm)

BET surface area (m2/ g)

Crystal size (nm)

Pristine TiO2 N-TiO2 Ho/N-TiO2-Ⅰ Ho/N-TiO2-Ⅱ Ho/N-TiO2-Ⅲ Ho-TiO2

0.89 1.18 1.30 1.55 1.54 1.47

94 116 128 135 147 132

25.7 21.3 12.6 14.8 10.5 16.1

Fig. 4. Raman spectra of the as-prepared pristine and doped TiO2 samples.

intensities of the peaks are well-consistent with the corresponding crystal size listed in Table 2. These results help confirm the presence of rutile TiO2 and the influence of dopants on the resultant crystal size. In order to investigate the chemical constitution of the Ho/N-TiO2Ⅲ sample, XPS spectra of N1s Ho 4d and O 1s were obtained, and the results are shown in Fig. 5 and Table 3. Assuming that the C 1s line lies at 286.6 eV, the calibration of the data was completed [18,41,42]. Therefore, the data provided in Table 2 do not perfectly match the counterpart spectra in Fig. 5 (a). Ti 2p1/2 and Ti 2p3/2 binding energies are located at 464.8 and 459.2 eV (space 5.6 eV) respectively, which are in accordance with the literature data for TiO2, and indicate that Ti mainly exists in the chemical state of Ti4+ on the surface [43]. The inset of Fig. 5 (a) is Ho 4d and Fig. 5 (b) shows the result of the O 1s fit spectra of Ho/N-TiO2-Ⅲ, indicating different oxygen environments,

Fig. 3. XRD patterns of the as-prepared pristine and doped TiO2 samples.

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Fig. 6. UV–vis absorption spectra of the as-prepared pristine and doped TiO2 samples.

Overall, results from this work attest that the hydrothermal method has successfully synthesized Ho and N co-doped TiO2. Furthermore, there is an apparent nascent synergistic effect existing between the doped Ho and N. The optical absorption properties of TiO2 nanocrystal, pristine TiO2, N-TiO2, Ho-TiO2 and Ho/N-TiO2 series catalysts were investigated by means of UV–vis absorption spectra, and the results are shown in Fig. 6. Then, the x-axis was changed to 1240/λ and y-axis was converting to (Ahν)2. The band gap is estimated with equation (αh ν) = k (h ν− Eg ) , in which α is adsorption coefficient, k is a constant, and Eg is the band-gap energy. And extrapolated intercept was the value of Eg. The values of band gap energy were listed in Table 4. Compared with the pristine TiO2, all the doped TiO2 samples exhibited higher absorbances in the range of 300 ˜ 525 nm. Furthermore, an obvious red-shift phenomenon was also observed in the absorption profiles of the doped TiO2 samples. It is attributed to the existence of surface oxygen defects due to the presence of enlarged TieOeTi bond angles and abundant coordinated unsaturated Ti atoms. In addition, the absorption edge of doping sample was gradually shifted to visible region instead visible shoulder overlapped pristine TiO2, which indicates that the incorporation of Ho lead to the appearance of energy level located down the conductive band. In the work by Zhao et al. [44], TiO2 nanocrystals with an oxygen vacancy need lower energy to excite photogenerated electrons from valence band to conduction band, indicating that the photons in the visible region are more likely to be absorbed. The absorption behavior of N-TiO2 is consistent with the previous report by Diwald that N-doped rutile had an increased absorbance in the range of 414 ˜ 517 nm (2.4 ˜ 3.0 eV). Thus it can be said that the N-doped TiO2 achieved photo-response ability in the visible light range, which resulted from the formation of a mid-gap in the band gap by the N dopant that was incorporated into the lattice. It has been demonstrated that the absorption spectra of TiO2 doped with rare earth metals present a red-shift band gap transition due to the charge transfer between the 4f-orbital electrons of the rare earth ion and the TiO2 conduction or valence band [31]. Thus, the enhanced visible light absorbance of Ho-TiO2 indicates that Ho-TiO2 achieved the visible light photo-response ability. All of the Ho/N-TiO2 samples gained higher absorbance and showed larger red-shifts in the visible light region than

Fig. 5. XPS survey spectrum of (a) the Ho/N-TiO2-Ⅲ sample; insets are the corresponding high-resolution XPS spectra of Ho 4d and N 1s. (b) high-resolution XPS spectra of O 1s. Table 3 XPS fitting data for the Ho/N-TiO2-Ⅲ sample. At. (%)

Ho/N-TiO2-Ⅲ

Ho 4d5/2

N 1s

Ti 2p

O 1s

0.64

0.13

32.21

66.96

and the peak at 530.8 eV should be attributed to the presence of the HoO-Ti bond [43]. It is reasonable to assume that Ho forms Ho-O-Ti bonds with broken Ti-O bonds during the hydrothermal process, which can affect the twisting or rotation of O atoms. Thus, Ho atoms were incorporated into the TiO2 lattice and doubtless formed chemical bond, instead of participating simple physical adsorption. The information provided from XPS corresponds perfectly to the results from XRD and Raman spectroscopy, which explain the effect of Ho doping on the structure and morphology of Ho/N-TiO2-Ⅲ. After Ho doping, the SBET of the samples have a general increasing trend. The increase of SBET is further evidence of the grain refinement due to Ho doping. The XPS spectra for the N 1s region of Ho/N-TiO2-Ⅲ and its fitted curve are exhibited in the inset on the left side of Fig. 5 (a). There is a broad peak from 397 to 403 eV, which may be attributed to the combined action of substitutional N displacing O in the form of OeTieN and the presence of oxidized or interstitial states such as TieOeNeO and TieOeN [29]. Based on the XPS fitting data summarized in Table 3, the stoichiometric ratios of the constituents of Ho/N-TiO2-Ⅲ are estimated to be: Ti0.966Ho0.019O2.011N0.004.

Table 4 Optical properties of the as-prepared pristine and doped TiO2 samples.

5

Samples

Band gap energy (eV)

Absorption edge wavelength (nm)

Pristine TiO2 N-TiO2 Ho/N-TiO2-Ⅰ Ho/N-TiO2-Ⅱ Ho/N-TiO2-Ⅲ Ho-TiO2

3.00 2.88 2.74 2.70 2.67 2.82

413 431 452 459 464 440

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those of the pristine and mono-doped TiO2, which can be ascribed to the formation of new and accessible electronic states in the band gap of TiO2 as a result of the Ho-N co-doping. It further indicates that the synergic effect between Ho and N can increase the visible light absorption intensity, which is favourable for improving the photocatalytic activity for CO2 reduction with H2O. Differing from the mono-doped TiO2, absorption peaks centered at about 480 nm were present in the spectra of Ho/N-TiO2. Herein, it is proposed that a possible charge transfer occurred in Ho/N-TiO2 between the 4f-electron of Ho and the mid-gap induced by the N dopant, with the Ho/N-TiO2-Ⅲ attaining the highest absorbance. The difference in the absorption spectra of the Ho/N-TiO2 samples can be attributed to the different amounts of N and Ho, as well the quantum size effect caused by the decrease of crystal size. The band gap energies of the as-prepared TiO2 samples were estimated by the Kubelka-Munk function [45], and the results are listed in Table 4. The results show that the band gap energies of the Ho/N-TiO2 samples were reduced to 2.67 ˜ 2.74 eV, which are apparently narrower than those of the pristine (3.0 eV) or the mono-doped TiO2 (2.88 eV, 2.82 eV). Consequently, the observed absorption features indicate that Ho/N-TiO2 achieves visible light photocatalytic activity; moreover, the visible light photocatalytic activity is higher than those of either the mono-doped NTiO2 or Ho-TiO2. Thus, Ho/N-TiO2 with enhanced visible-light absorption can be considered a promising photocatalyst for CO2 conversion with H2O. It should be noted that many studies on TiO2 film have emphasized the significance of water absorption, because it plays an extremely important role in the reaction. On the one hand, when H2O is the only reducing agent in the photoreduction of CO2, it is noticeable that the H2O is the unique supplier of hydrogen to provide H2. After water absorbing onto the catalyst, the H2O molecule is first dissociated into H+ and OH−, and the hydrogen atom is produced via the reduction of H+ by accepting an electron [46]. Meanwhile, the OH− combines with a hole and becomes an ·OH radical, which is regarded as the dominant active species in photocatalysis [24]. Therefore, wettability is an important parameter for TiO2 materials because a hydrophilic surface means more water adsorption, which is beneficial to photoreduction. Fig. 7 shows water droplets on pristine TiO2 and Ho/N-TiO2-Ⅲ, respectively. In Fig. 7(a), the contact areas nearby the outermost threephase contact lines are in the Cassie-Baxter state, with corresponding apparent contact angle of 53°. Ho/N-TiO2-Ⅲ, by contrast, is more hydrophilic, with its apparent contact angle decreasing to 7°. Obviously, Ho/N co-doping treatment can significantly enhance the hydrophilicity. The enhancement of hydrophilicity is due to two reasons: (1) the sample morphology [47,48]. The pristine TiO2 nanorods are cubic columns with quasi-square tops and smooth side surfaces. By contrast, Ho/N-TiO2-Ⅲ is of cone-like nanorods with sharp tops. This remarkable change of morphology leads to a much smaller surface contact area

Fig. 8. FT-IR spectra of the as-prepared pristine and doped TiO2 samples.

with droplets; (2) the chemical constitution. The doping Ho influences the charge balance of the sample, so that more OH− would be adsorbed onto the catalyst for charge balance [49]. Meanwhile, these OH groups anchor ambient water molecules to form strongly bound OH-H2O complexes that act as nucleation centers for further water adsorption (details are discribed in the FTIR section) [50]. As a result, Ho/N-TiO2Ⅲ possesses much more eOH which can boost the spreading of the water droplets. In addition, the improvement of hydrophilicity can consistently provide adequate eOH groups for photocatalysis. As discussed above, the amount of water adsorption is an important step in the photocatalysis of CO2. Similarly, the hydroxyl group, as an intermediate, is another key factor to which we should pay attention, because its quantity directly determines the number of hydroxyl radicals. Other chemical groups on the TiO2 surface as well as hydroxyl groups are susceptible to doping treatment. Fig. 8 shows the FTIR spectra of the pristine and doped TiO2. For all samples, the peak centered at about 3410 cm−1 can be assigned to the OeH stretching vibrations of hydroxyl groups [50,51]. The peak situated at 1620 cm−1 originated from the HeOeH bending vibrations of surface-adsorbed water [52], and the broad peak ranging from 400 to 800 cm−1 arose from the Ti-O stretching vibrations of TiO2. In addition, the peak at approximately 1050 cm−1 presented in the spectra for Ne and Ho/N doped TiO2 is the characteristic vibration of the NeO group [12]. Compared with the pristine TiO2, the intensities of the OeH and HeOeH bands showed apparent increases for all doped TiO2 samples, especially the co-doped TiO2. As the −OH and the HeOeH groups on TiO2-based materials can react with the photogenerated holes during photocatalysis, in general, a moderate increase

Fig. 7. Water contact angles of (a) the as-prepared pristine TiO2, (b) Ho/N-TiO2-Ⅲ samples. 6

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Fig. 10. Evolutions of CH4 products and the selectivity of CO2 conversion of CO2 with H2O over N-TiO2, Ho-TiO2 and Ho/N-TiO2 series catalysts. Fig. 9. Photoluminescence (PL) spectra of the as-prepared pristine and doped TiO2 samples.

3.2. Photocatalytic activity for CO2 reduction of photocatalysts The photocatalytic performances of N-TiO2, Ho-TiO2 and Ho/NTiO2 series catalysts for CO2 conversion with H2O were carried out in a gas-tight circulation system under visible light illumination (details are given in 2.4 section). The corresponding evolved evolution amounts of methane are shown in Fig.10. From Fig.10, the amount of methane over TiO2 after doping treatment is notably much larger than that of the pristine one, demonstrating that doping, and especially co-doped, is undoubtedly a promising modification method. Moreover, the methane yields of the Ho/N-TiO2 samples were notably higher than those of the mono-doped N-TiO2 and Ho-TiO2. The NTiO2 sample showed the lowest CH4 yield, with a mean rate of 5.6 μmol/gcatal./hour, resulting from the low absorbance and the relatively high recombination of the photo-generated charge carriers. The Ho-TiO2 sample obtained a mean rate of CH4 yield of 8.0 μmol/gcatal./ hour, while, Ho/N-TiO2-Ⅲ exhibited the highest CH4 yield with a mean rate of 29 μmol/gcatal./hour, which can be ascribed to the highest absorbance and the lowest recombination of the photo-generated charge carriers among the various catalysts. Compared with N-TiO2 and HoTiO2, the mean rate of CH4 yield of Ho/N-TiO2-Ⅲ was increased by 4.2fold and 2.6 fold, respectively. This indicates that the noticeable synergistic effect of Ho and N can enhance the photocatalytic performance for CO2 conversion with H2O to methane. Among the Ho/N-TiO2 samples, Ho/N-TiO2-Ⅲ possessed the best performance of photocatalytic reduction of CO2 under visible light illumination, and it is proposed that by appropriate co-doping with Ho and N, Ho/N-TiO2 can reach an optimal state, balancing the absorbance and recombination of photo-generated charge carriers with the changes of BET surface area, crystal size and chemical constitution. Although we concentrate herein on production of CH4 and CO, methanol is still an important product of photocatalytic CO2 conversion. The inset of Fig.10 shows the types and amounts of CO, methane and methanol products of pristine TiO2 and Ho/N-TiO2-Ⅲ. It is also generally known that photocatalytic reduction of CO2 to methane and methanol takes place accompanying the reduction of H2O to H2 [54]. However, at this time we are focused on methane and methanol, and that is the reason that H2 in not indicated in this figure. According to stoichiometry, for the number of electrons consumed in the formation of the reduced products, one unit of CO is equivalent to 1/4 unit of CH4, and similarly, one unit of CH3OH is equivalent to one unit of CH4. Here, the equivalent total yield of reduction products can be approximately calculated by the following formula: n(total) = n (CH4) + n(CH3OH) + 1/4n(CO). Compared with the pristine TiO2, the CH4 total yield of doped TiO2 remarkably increased under UV illumination (Fig. 6b), in particular, the yield of Ho/N-TiO2-Ⅲ was approximately 4 times higher than that of pristine TiO2. It needs to be stressed that the constitutions of reduction products of CO2 photocatalytic reduction showed significant differences between the pristine TiO2 and

of the −OH and the HeOeH groups is crucial for the TiO2-based materials. The increase of intensities of the −OH and the HeOeH groups in co-doped TiO2 can be attributed to the combination of the presence of Ho3+ [30], the NeO group and the oxygen vacancies induced by the doping treatment [29]. It indicates that the synergic effect between Ho and N in the co-doped catalyst can increase the hydroxyl group adsorption capacity of the catalyst surface. The possible reason for this is that Ti4+ ion replaces Ho ion with a +3 oxidation state and creates a charge imbalance [43]. The charge imbalance must be satiated, so more hydroxide ions would be adsorbed onto the surface for charge balance. In the meantime, the doping N may enlarge the imbalance even further. To investigate the charge transfer behavior of the photo-generated charge carriers in the as-prepared samples, their corresponding photoluminescence (PL) spectra were obtained, as shown in Fig. 9. Generally, the lower the intensity of the PL-spectra, the better the ability of the photocatalyst to suppress the recombination of photo-generated charge carriers [53]. Both the pristine and doped TiO2 samples possess PLemission peaks in the region of 400˜500 nm. Compared with the broad, intense PL-emission peak of the pristine TiO2, the PL-emission peaks of the doped TiO2 not only become narrower, but also exhibit obvious redshifts in their locations. Significantly, the intensities of the PL-spectra of the doped TiO2 are considerably reduced in comparison with that of the pristine TiO2, which suggests that the recombination of photo-generated charge carriers was efficiently suppressed by the doping. As aforementioned, the doped TiO2 had an increased amount of surfaceadsorbed OH−, which may react with the photo-generated holes to suppress the recombination of photo-generated charge carriers. The intensities of the PL-spectra of the studied samples decreased in the following order: pristine TiO2, N-TiO2, Ho-TiO2, Ho/N-TiO2-Ⅱ, Ho/NTiO2-Ⅰ and Ho/N-TiO2-Ⅲ. Notably, the intensities of the PL-spectra of the Ho/N-TiO2 samples were obviously further decreased, and it is proposed that a possible synergy effect induced by the Ho and N codoping enhanced the suppression of the photo-generated charge carriers in Ho/N-TiO2. Although the Ho/N-TiO2-Ⅱ sample was prepared with the highest concentration of dopants, the intensity of its corresponding PL-spectra was unexpectedly higher than those of both Ho/NTiO2-Ⅰ and Ho/N-TiO2-Ⅲ. This increase of the intensity of the PLspectra of Ho/N-TiO2-Ⅱ can be ascribed to the partial dopants becoming the recombination centers of the photo-generated charge carriers, and the augmented crystal size reducing the charge transfer. Therefore, Ho/ N-TiO2-Ⅲ showed the lowest PL intensity, indicating that the Ho and N co-doping can improve the separation of photo-generated charge carriers. Integration of the above UV–vis absorption spectra indicated that Ho/N-TiO2-Ⅲ showed the highest absorbance and the lowest PL intensity, suggesting that the synergic effect between Ho and N doping in TiO2 can increase the separation efficiency of photogenerated electrons and holes, which is beneficial for CO2 reduction with H2O. 7

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Fig. 11. Schematics of (a) the decrease of band gap of TiO2 by co-doping with Ho and N (b) the photocatalystic reduction mechanism for Ho/N-TiO2 photocatalyst.

depicted in Fig. 11. As shown in Fig. 11a, the Ho/N-TiO2 photocatalyst achieves a narrowed band gap. The N that is incorporated into the lattice can induce a new N 2p mid-gap slightly above the original O 2p valence band of TiO2, and thus the band gap decreases. When Ho is introduced, charge transfer will occur between the 4f-electron of Ho3+ and the conduction or valence band (both the O 2p and the N 2p for Ho/N-TiO2). Hence, the Ho/N-TiO2 photocatalyst can be activated by visible light. The charge imbalance caused by the doping of Ho and N is neutralized by the formation of oxygen vacancies in Ho/N-TiO2, leading to the increase of surface-adsorbed OH−. Upon irradiation, the excited Ho/N-TiO2 photocatalyst gives birth to the e−-h+ pairs, and the photo-generated holes prefer to react with the surface-adsorbed OH− to produce oxidative hydroxyl (·OH) radicals, facilitating the separation of photo-generated e−-h+ pairs (Fig. 11b). The hydroxyl groups and the remaining electrons can then execute photocatalytic redox reactions such as the reduction of CO2.

doped TiO2. The pristine TiO2 had reduction products composed of CO and CH4, while on the contrary, CO and CH3OH were the main products for Ho/N-TiO2-Ⅲ. In this system, CO2 photocatalytic reduction adopted the carbene pathway. With the CO2 reduction proceeding, CH· radical, carbine and methyl radical (CH3·) form sequentially. When the system has a relatively low amount of water or poor hydrophilicity, CH3· prefers to react with H· to produce CH4, leaving CH4 as a main reduction product. As the Ho/N-TiO2-Ⅲ was rather hydrophilic, CH3· tends to combine with ·OH to form CH3OH, leading to the pronounced increase of CH3OH in the reduction products. This indicates that the synergic effect between Ho and N in TiO2 can enhance the photocatalytic conversion with H2O to methane and methanol. Considering that the photocatalytic CO2 with H2O reduction to methane is a competitive reaction with CO2 reduction to methanol, we can control the amount of the doping elements in order to regulate the variety of the products. We hold the opinion that the product regulation process should be further studied. It is a boon for researchers who are interested in TiO2 photocatalysis, especially with co-doping.

4. Conclusions Ho/N-TiO2 was successfully fabricated by hydrothermal treatment, and showed high performance in CO2 photocatalytic reduction under visible light illumination. With the introduction of Ho and N in substitutional states, Ho/N-TiO2 can extend absorption to the visible light region. Meanwhile, the crystal size of Ho/N-TiO2 decreased in comparison with that of pristine TiO2. An increase of the amount of surfaceadsorbed OH− on Ho/N-TiO2 promoted the reaction of OH− and photogenerated holes, which efficiently suppressed the recombination of photo-generated charge carriers. Ho/N-TiO2 reduced CO2 via the carbene pathway to obtain reduction products mainly consisting of CH4 and CO. It was also found that the nature of the photoreduction products could be regulated by controlling the relative amounts of doping elements in the Ho/N co-doped systems.

3.3. Mechanism of charge transfer in photocatalysts The nature of the photocatalytic CO2 conversion with H2O under light illumination is a solid-gas heterogeneous catalysis reaction driven by photogenerated charges. It is well-known that the CO2 reduction to methane and methanol must be confirmed by energy band theory, considering the positions of the conduction band(CB) and valence band (VB) of the photocatalysts and the corresponding reduction potentials of reactions. When the reduction potential of the reaction is below the CB position of the photocatalysts, photogenerated electrons can be employed efficiently. In the period of reaction, the possible reactions are described by the thermodynamic reduction potentials converted to SCE reference(pH 7), and equations are listed in Table 1 [53]. As the base of photogenerated electrons, the CB potential of TiO2 (-0.8 V vs. SCE at pH 7) is obviously more negative than the reduction potentials of E θ (CO2/CO)(-0.77 V), E θ (CO2/CH4)(-0.48 V) and E θ (CO2/CH3OH)(-0.62 V), indicating that the photoreduction of CO2 to CO, CH4 and CH3OH are all theoretically achievable. As reported by Wei at el. [53], the surface density of active species can change the evolution rules of products, which is in perfect agreement with the theoretical and experimental analysis that Ho/N-TiO2 possesses more hydroxyl radicals on the surface. After analyzing the photoreduction process of CO2, it is apparent that it can be divided into three steps: firstly, band-gap excitation stimulates the generation of electron-hole pairs; secondly, the pairs are separated and gather on the surface; thirdly, CO2 and the adsorbed H2O react with photogenerated electrons. Thus, light-harvesting, adsorption and activation for CO2, and electron-hole pair separation efficiency are of great importance in enhancing photocatalytic activity. Based on what has been discussed above, the photoreduction mechanism for CO2 with H2O to CH4 over Ho/N-TiO2 is proposed and

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